It is known, e.g. from U.S. Pat. No. 3,642,597, so coat such a body by immersing same in a carrier liquid of isopropanol, ethyl acetate, methanol or de-ionozed water, for example, in which particles of a finely comminuted passivating substance such as glass are suspended. Electrophoretic deposition of the particles on the substrate, i.e. the semiconductive body, is facilitated by bubbling ammonia through the liquid to activate the solution by placing a positive surface charge on these particles.
With passivating materials rich in metal oxides, such as ZnO, their interaction with ammonia results in the release of metal ions which are attracted by the negatively polarized substrate together with the suspended coating particles and tend to remain on the semiconductor surface when the particles are subsequently fused into a continuous layer. Their presence in or on the semiconductive body impairs its performance, as by increasing the leakage current of a reverse-biased diode.
The problem referred to may be somewhat alleviated through the use of glasses low in zinc oxide, yet practical experience has shown that the formation of metallic cations cannot be completely prevented in a bath of this type. It is apparent that the difficulty could be avoided by applying a negative rather than a positive surface charge to the particles, but no additive known up to now could serve this purpose.